1. Field of the Invention
This invention relates to a method and apparatus for detecting and analyzing gear defects.
2. Related Art
Several attempts are disclosed in the prior art literature to analyze vibrational systems. Many of those attempts are highly complex and limited in their capability.
U.S. Pat. No. 4,520,674 entitled VIBRATION MONITORING DEVICE is typical of recent prior art systems. A signal generated by a vibration monitor is initially processed by a signal conditions module which includes anti-aliasing filters to enhance the accuracy of the data collected. Further preprocessing is performed by a multi-function module which also increases the speed and reliability of the system. The data is subsequently analyzed by a microprocessor and displayed if desired on a monitor.
U.S. Reissue Pat. No. Re. 31,750 entitled DATA ACQUISITION SYSTEM is similar to the foregoing concept. Signal information is brought into a multi-channel multiplexer. Signal pick up is performed by self-amplified accelerometers. The system subsequently performs trend analysis on the historical data which consists of representative amplitudes of stored electrical signals.
U.S. Pat. No. 4,429,568 entitled ACOUSTICAL DEFECT DETECTION SYSTEM describes an invention in which the incoming signal is also preconditioned. The preconditioning is performed through the use of amplifiers, a high pass filter, a low pass filter, a full wave rectifier and an analog/digital converter.
U.S. Pat. No. 4,574,633 describes APPARATUS FOR DETECTING TOOL DAMAGE IN AUTOMATICALLY CONTROLLED MACHINE TOOLS wherein historical data is compared to present data.
The following prior art patent references describe possibly relevant systems for detecting the deterioration of gears. U.S. Pat. Nos. 3,758,758; 3,842,663; 4,335,612; 4,550,603 and 4,550,604. Certain of the foregoing patents relate to specific types of gears. For example, U.S. Pat. No. 3,758,758 relates specifically to the meshing of helicopter gears and U.S. Pat. No. 4,335,642 relates to a method of detecting irregularities on beveled gears. U.S. Pat. Nos. 4,550,603 and 4,550,604 both relate to gear inspection techniques.
Lastly, the prior art patent literature describes other systems of lesser relevance. For example, U.S. Pat. No. 4,352,293 includes a detailed description of Fourier analysis. U.S. Pat. No. 3,913,084 includes a discussion of the use of accelerometers in the context of noise detectors and analyzers. U.S. Pat. No. 3,694,637 describes a relatively simple system for analyzing wear on a tool in which a minicomputer is used for the purpose of analyzing the ultimate results.
A common method for performing vibration (or acoustic, torque, or force) based analysis of operating gear systems involves use of spectrum or cepstrum analysis instrumentation. This instrumentation allows amplitude estimation of vibrations related directly to gear tooth meshing frequencies. These amplitudes are then compared over time or between like machines, and conclusions on gear status are reached. Such methods give overall measures of gear performance and are not very sensitive to defects localized on the gear, and are subject to background interference. In addition, observations over time, or comparisons between similar machines is required to interpret these measures. The present invention describes a method for obtaining more detailed gear condition information on a tooth-by-tooth basis, and allows comparison among the teeth for interpretation. Disclosed are methods to extract the tooth-by-tooth information in the presence of numerous forms of interference, and to allow construction of a gear diagnostic image for further examination.
Another approach, explored specifically in the helicopters preventive maintenance industry, involves time domain averaging of vibration signals and computing statistical measures such as the fourth and sixth moment, from these time domain averages. Changes in these measure over time is the basis of the approach. This technique does not produce a tooth-by-tooth level of analysis and is subject to interference, thereby limiting its sensitivity and reliability in such applications.
Insofar as understood, none of the prior art taken individually or in combination teach or suggest the present invention which includes, among other things, the capability of analyzing tooth-to-tooth interactions over a long period of time, and then stacking the interactions in such a way as to make a gear defect detectable.